Rigid Body World (Groups) Explained
The rigid body world contains global settings for the rigid body simulation.

With the "Rigid Body World" checkbox you can simply disable/enable the rigid body simulation. 

In order to understand the groups, you have to understand the following:


The rigid body system uses regular Blender groups to keep track of the objects that need to be simulated. 
When you add rigid body physics to the first object, a group called "Rigid Body World" is automatically created, and the object is added to it. You can see it in the Outliner, if you set it to show groups.
When you add rigid body physics to other objects, they are also added to the "Rigid Body World" group
You can create your own groups, and select them in the "Rigid Body World" panel. In this case the objects belonging to the new group will be simulated and the objects belonging to the old group will be ignored.